Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
An example composite membrane may include a thin active (selectively permeable) layer, e.g., porous graphene, on a porous (broadly permeable) membrane support. Composite membranes may be prepared with thin active layers, for example, ranging from about 500 angstroms to about 1 micrometers—as thin as possible, since resistance to gas or fluid flow may scale linearly with membrane thickness. Such thin films may generally be fragile when handled in isolation, so composite membranes are often formed by casting the active layer from solution directly on the support.
High quality monolayer graphene is generally insoluble and may be difficult to cast evenly onto a substrate from solution. Instead, mechanical transfer techniques may be employed to transport graphene from a growth substrate (for example, copper or nickel) onto a porous membrane support. However, adhering the graphene to the porous substrate may be difficult because the graphene sheet may be suspended over the pore voids and not in contact with the porous membrane support material. This challenge may be magnified since bare, nonpolar graphene may bond to the support through just van der Waals forces. Good adhesion of the graphene to the support may be critical to permit handling of the membranes during manufacturing and to ensure reliability in use.
Further, it may be desirable that any given membrane material may sustain the transmembrane pressure occurring during separation. This transmembrane pressure may range, for example, from one atmosphere for CO2 scrubbing in coal plants, up to 100 or more atmospheres for natural gas sweetening or reverse osmosis. This transmembrane pressure may deform the active layer of the membrane into the pores of the support, by which the membrane may be subjected to significant tensile stress.
With larger pore size and/or higher pressure, the active region of the membrane may burst, damaging the membrane's separation properties. However, the incorporation of a membrane support with small pores may restrict the flow of fluid substantially, and may limit the total overall separation flux through the composite membrane. It may be desirable to employ a membrane support with pore sizes as large as possible to avoid unnecessarily restricting flow. An example of a common support is polysulfone with nano or micro scale pores, since it has tensile strength useful for good mechanical support, and may be formulated with fairly uniform pore sizes. However, supports may be quite expensive: at roughly $50/m2, polysulfone may be the greatest cost component of a composite membrane which may limit membrane applications. In particular, graphene monolayers at one atomic layer thick may add little cost while providing permeabilities that greatly outstrip the best existing polymeric membranes today. In graphene composite membranes, even polysulfone supports may undesirably restrict flow and/or raise cost.
The present disclosure appreciates that preparing composite membranes including porous graphene layers, e.g., for use in separations, may be a complex undertaking.